NEIGHBOURHOOD OF SARN, CAERNARVONSHIRE. 451 



church, on the east side of the diabase-ridge, and can be traced 

 southward to the coast at Tyn-y-borth, being quarried at Treheli. 



The microscopic study of these rocks reveals features of consider- 

 able interest, and they will accordingly be treated in rather more 

 detail than the foregoing rocks. Taking the hornblende-diabases of 

 the district as a whole, the original constituent minerals are apatite, 

 magnetite, picotite, ilmenite, felspar, olivine, augite, and hornblende. 

 Among the secondary products we find in various slides leucoxene, 

 kaolin, serpentine, magnetite, actinolite, a chloritoid substance, a 

 radiating zeolite, calcite, and quartz. 



Apatite occurs but rarely, in large cross-jointed prisms. Original 

 magnetite grains are frequent, and belong to an early stage of the 

 consolidation. A few rounded grains, brown and slightly translucent, 

 which accompany the magnetite, are referred to picotite. Ilmenite, 

 in skeletons of intersecting rods, is found in one variety, otherwise 

 abnormal. On the whole these rocks are poor in accessory minerals. 



A felspar of the plagioclase series is always abundant in good 

 crystals, showing the usual forms. The crystals are sometimes 

 simple, generally twinned once or twice, sometimes finely lamellated, 

 and very rarely showing a cross-twinning corresponding to the 

 pericline-law. The extinction-angles are usually nearest to those of 

 labradorite, but some symmetrical sections extinguish at 40° from 

 the twin-plane, indicating anorthite. Only one rock, that of Mynydd 

 Penarfynydd, shows in places two distinct generations of felspar. 

 With this exception the felspar is always of earlier formation than 

 any of the augite or hornblende. 



Olivine has probably been an occasional constituent of the horn- 

 blende-diabases here as elsewhere, but it is not now detected in any 

 of the slides, and serpentine-grains which seem to result from it are 

 not often seen. There is, indeed, plentiful serpentine in many of the 

 slides ; but in some cases this substance clearly results from the 

 alteration of hornblende, in others it probably comes from a rhombic 

 pyroxene, while the mesh-structure so characteristic of pseudomorphs 

 after olivine is rarely indicated. 



The augite is found either in good crystals or in ophitic plates and 

 shapeless grains, the latter mode of occurrence being by far the more 

 common. When crystals occur, they show in cross-section the usual 

 octagon, the pinacoids being rather more developed than the prism- 

 faces, while the terminal planes seem to belong to (111) and (001). 

 The extinction-angle in a section parallel to the clinopinacoid is 39° 

 or 40°. The prismatic cleavage is constantly well marked, and in a 

 few cases pinacoidal cleavages were noticed, but never any diaUagic 

 structure. The augite is colourless or extremely pale brown in thin 

 sections : only in one rock (summit of Mynydd-y-graig) is this seen 

 to pass into augite of a deeper violet-brown with slight pleochroism. 

 This and the occasional hour-glass structure recall the augite of some 

 of the teschenites. Twinning on the usual law is sometimes ob- 

 served, the orthopinacoid being twin-plane and face of composition. 



The hornblende rarely forms imperfect crystals, contained by the 

 clinopinacoid and prism-faces, without terminations : in the large 



